Talk-back communication system



Sept. 9, 1941. F. w. NICKERSON ETAL TALK-BACK COMMUNICATION SYSTEM FiledOct. 27, 1959' 6 Sheets-Sheet l lvlcke son 2m 0- Moore Sept. 9, 1941. F.w. NICKERSON ETAL 2,255,517

TALK-BACK COMMUNICATION SYSTEM Filed 001;. 2'7, 1939 Sheets-Sheet 2ATTORNEY.

%/+erbe f rneH- Sept. 9, 1941. F. w. NICKERS ON ET AL TALK-BACKCOMMUNICATION v SYSTEM Filed Oct. 27, 1939 6 Sheets-Sheet 3 Sept, 19430F. w. NICKERSON ETAL. 2,255,517

TALK-BACK COMMUNICATION SYSTEM Filed Oct. 27, 1939 6 Sheets-Sheet 4 f inw/sec.

Fr ed Sept. 9, 1941. w NlCKERSON HAL 2,255,517

TALK-BACK COMMUNICATION SYSTEM Filed Oct. 21 19.39 6 Sheets-Sheet 5 a M.3 .234 mucv uwsw w 3 d 1. 2 vzoio i m I A 7' TORNE Y5 Se t. 9; 1941.

F. 'w. NICKERSON ETAL TALK-BACK COMMUNICATION SYSTEM 6 Sheets-Sheet 6Filed Oct, 27, 1939 02? x. Zin -i view ENTOR..

5 n mm w mwmw N C r w m wm T dmw T A Fre Wi l $5 ?aiented Sept. 9, 1941TALK-BACK COMMUNICATION SYSTEM Fred W. Nickerson and William C. Moore,Brooklyn, and Herbert Barnett, Jackson Heights, N. Y., assignors toGuided Radio Corporation, New York, N. Y., a corporation of :New YorkApplication October 27, 1939, Serial No. 301,546

16 claims ,(01. 119-1) This invention comprises a combination ofinstrumentalities associated to form an audio-frequency communicationsystem capable of sound transmission in either direction.

In a broad sense, the inventionrconsists of a combination of electricalinstrumentalities of such characteristics'and so associated that,at

one end of the system there is provided a transmitter or microphone anda receiver or loudspeaker, while at the other end of the system thereare provided a plurality of spatially distributed combined microphonesand loudspeakers or transmitters and receivers so that at the latter endtalk-out as well as talk-backcan be accomplished..

An important object of this invention is to provide a system of theabove, nature having characteristics such that the combined loudspeakerand microphone, which hereinafter may be termed transducers, are capableof picking up and transmitting intelligible sounds from substantialdistances. Another object of this invention is to provide a system ofthe above type which may be turned on from either end so that speech maybe transmitted from either end to the other under controls at one end.

These and many other objects which will become apparent from thefollowing detailed de-. scription of several forms of the invention aresuccessfully secured.

This invention resides substantially in the combination, construction,arrangement and relative location of parts, all in accordance with thisdisclosure.

- In the accompanying drawings,

Figures 1 and 2 together comprise a diagrammatic illustration of oneform of system in accordance with the objects and principles of thisinvention;

Figure 3 is a diagrammatic view of a modified form of system likewise inaccordance with the objects and principles of this invention;

Figure 4 is a graph illustrating the response curve of the transducers'showing'a rising response with increasing frequency;

Figure 5 is another chart illustrating the directionalcharacteristics'of the transducers;

Figure 6 is a chart of the response of the electrical transmissionnetwork and transducer in combination illustrating its non-linearresponse;

Figure '7 is a chart illustrating the response of the transducers asloudspeakers, upper curve, and microphones, lower curve; and

Figure 8 is a chart illustrating the sensitivity of the human car atvarious frequencies.

In order to fully appreciate the nature of this invention and the mannerin which the objects thereof are accomplished, a brief discussion of theprinciples thereof is desirable before describing examples of the formswhich the complete system may take. The system herein described willeffect two-way longdistance audio communication with intelligible signalpickup over'distance's heretofore unattained: the same electro-acousticdevice as a microphone for speech pickup as is used forreproducing-;speech as a loudspeaker. .In this .twg way'j'ailidiofrequency communication system,"amplifiergunits are employed which arecontrolled from one end of the system so that when the transducer isacting as -'a microphone it is connected tdthein'put of'the amplifyingsystemandwlien it isac ting 'as a "20 "loudspeakeritis connected.totheoi1tput or the amplifier.

The system ofthisinventionlutilises acombination of acoustictransdu'ce'rs;transmissionnetworks and amplifiers having elctricalandacoustical characteristics that would The normally commonly known inthat each is intentionally 'made to have a non-linearamplitude-frequency response and a limited frequency'range.1 Inaddition, there is provided in the system a so-called talk-back volumecontrol to permit reduction of volume to increase the effectiveness ofsound pickup at long distances.

These apparently low fidelity? components referred to are combined in asystem in ways which will be hereinafter described to efiect unusuallylong distance pickup of voice and. other sound signals by theelectro-acoustic transducers which are so designedjas to operateeficiently as high power loudspeakers. By high power is meant in thisdisclosure the capability of handling energy in amounts of the order often watts or more of electrical power input. 1 To illustrate theeffectiveness of systems inaccord- 'ance with this invention, itis notedthat the 55" much as 2500 feet, that is to'say," the speaker -the bestoperation as a pickup device.

may stand that far from the transducer and talk with a person at theother end of the sys tern. that no system heretofore known has beencapable of such operation, it being necessary es pecially where theloudspeakers are also used as microphones, to stand directly in front ofthem in order to transmit intelligible, speech. The result is thatsystems of this type may be installed, for example, on shipboard withthe main station on the bridge of the ship and the over the ship. Anoflicer on the bridge may with such a system communicate with any memberof the crew, who need not be anywhere near the transducers in order toreply. This result is the more remarkable because intelligible speechcan be transmitted through a substantial background of undesired sounds.

As will appear as the disclosure proceeds, the

combination into a communication system of the audio-frequency type ofcomponents having non-linear response and limited frequency range wouldbe intentionally avoided by skilled designers of conventionalloudspeaker communication systems for the reason that a result exactlycontrary to applicant's is normally striven'for, that is, high fidelityof sound transmission and reproduction.

It is, of course, recognized that a loudspeaker as a talk-backmicrophone is in itself not new, such use thereof having been heretoforesuggest-ed. When used, however, as a microphone such loudspeakers areonly capable of reproducing intelligibly sounds picked up in theimmediate vicinity thereof. The practical distinguishing feature inover-all result of applicant's system is its ability to intelligiblyreproduce sounds from unusually long distances.

Referring more in detail to the components of applicants system,reference is firstmade to the talk-back loudspeaker, i. e., thetransducer. In accordance with this invention, the transducer must haveinherent low frequency discrimination and high sensitivity in the regionof optimum ear sensitivity. To obtain such results the average trend ofthe frequency response curve of this device must rise with increasingfrequency, and between, for example, 200 and 2500 cycles the rate ofrise must not be less than three decibels per octave. Such a-responsecurve is illustrated in Figure 4. The straight line of this curveindicates the minimum average rising trend required for effectiveoperation while the other curve is a sample loudspeaker curve that meetsthis limit. Such a device when used as a loudspeaker permits efficientreproduction of sound and when used as a microphone provides Asillustrated in Figure 4, the frequencies below the important speechregion, that is, below about 200 cycles, are rapidly attenuated. This isdesirable because the low frequencies apparently produce a secondaryeffect which has a tendency to mask the higher and more important speechfrequencies and thus operate to reduce the intelligibility.

This device in accordance with this invention is also designed tohavestrong spatial discrimination. In other words, the device when used bothas a talk-out and a long distance talk-back unit must have a sharpdirectional effect in a dimensional sense.

Figure 5. The transducers should have a -di-' It will, of course, berecognized at once- The directional discrimina-- tion of a suitabletransducer is illustrated in I rectional characteristic greater thanthat illustrated in Figure 5 at a frequency of 1000 cycles and'should beso designed so as to have i an increasing directionaheffect up'to atleast a 5 frequency of 3000 cycles. Such-a device when operating eitheras a talk-out' or a talk-back unit permits the collection of sound in a'narrow cone along the axis in which the horn thereof is directed with amuch greater sensitivity to sounds within that region than to' sounds]and various transducer stations distributed at desired points noisesoutside of it. i v

The transmission circuits which receive the. electrical currentsgenerated by the pickup devices feed into an amplifier.

cally discriminate so as to attenuate the low and other undesirablefrequencies while increasing or emphasizing those frequencies whichinclude the vowel sounds, voiced consonants and such sibi- 1am; soundsas are desired for good intelligibility of speech. 'Like thetransducers, the frequency response of transmission circuits andamplifiers must have a trend which rises with increasing frequencybetween the lowest frequency trans- 5 mitted by the talk-back speakerand a frequency obtain the proper emphasis when the talk-back unit isused as a loudspeaker, the circuits are arranged so that the responsecurve of the electrical system may be automatically changed in anypredetermined manner but preferably so that frequencies below 200 cyclesper second are rapidly attenuated.

The sounds that the talk-back loudspeaker or transducer pick up are fedthrough the amplifier and reproduced by a receiving loudspeaker. Thisloudspeaker, which would be the one on the bridge in the case of shipinstallation, is not used in the'system shown, as a microphone. It must,

,however, be so designed that its. response is greatest in the highervowel voiced cojnsonant and sibilant regions, thereby .having itsgreatest efficiency in a band of frequencies starting not higher than1000 cycles up to not lower than 4000 cycles, and so constructed thatfrequencies below 500 cycles are markedly attenuated. The

response curve of such a loudspeaker is illustrated in Figure 6. Whendesired, a receiving loudspeaker of the conventional type may beemployed if used in conjunction with an electrical network havingtransmission characteristics such that the response curve of thecombieration as against talk-out operation. The earis more sensitive inthe higher vowel, voiced consonant, and sibilant regions at lowintensities so that it is important to keep the total volume of speechand noise low enough so that the car operates at a maximum sensitivity,as is illustrated as being necessary by the chart of Figure 8.

It is a well known fact that the sensitivity of the ear varies with theamplitude of the sound 0 impinging thereupon, and such a marked decreasein the ear sensitivity occurs if the volume is too greatly increased, aswhen an appreciable amount of extraneous noise is amplified, that "theeffectiveness of the talk-back reception is seriously impaired. Thisresult is normally un- The circuits just 5 ahead of the amplifier aredesigned to electrl-f 'lieved, hasnot heretofore been possible.

. 2,255Tsi7 derstood to be just opposite to what would be bridge thosetransducers which are to be operated. The transducers are indicated bythe 'ref erence numerals l2, there being one illustrated for each switchl2. While, of course, only three transducers are shown it is apparentthat any desired number as required may be employed.

Thev reference numeral I3 is used to indicate generally the controlunits, one of which is provided for eachtransducer and is controlled atthis directional efl'ect increases markedly as the frequency increases.This property combined with the low frequency discrimination of thetalk-back loudspeaker increases the effect ofhigher sensitivity in theregion of best ear sensitivity over what would be obtained if theincreased directional effect with frequency were not present.

Referring to Figures ,1 and 2, a description will be provided of asystem in accordance with the objects and principles of this invention.These two figures are to be read together, representing a singlecomplete system. It may be noted that one of the more suitable fields ofuse for this invention is on shipboard. The power supply for the systemof the invention is from the 110 volt direct current circuit of theship, represented by the reference numerals l and 2'. They are connectedthrough a main switch 3 and a pair of choke coils 4.and 5 to the supplyleads I and 2 for the apparatus of this invention. These leads arebridged by a pair of condensers 8 and l in series having their commontap grounded, as indicated at 8. The combination of choke coils andcondensers provides a filter circuit for keeping out any undesiredcurrent fluctuations which may be present in the power circuit feedingthe apparatus.

In the various figures the power supply leads i and 2 are not extendedto all of the various instrumentalities receiving energy therefrom forpurposes of simplifying the drawing. However.

thecorresponding leads of the various instrumentalities are providedwith the same reference numerals l and 2, from which it will be apparentto those skilled in the art that all of the leads marked l are connectedto the power lead 8 and all of the leads 2 are connected to the powerlead 2.

' At 9 and H] are illustrated the loudspeaker and microphone, which inthe case of a ship installation would normally be on the bridge of ashipunder the control of an officer. As noted in ing type. This switchis likewise on the bridge adjacent to the microphone it) so that whenthe bridge oflicer wishes to use the system to talkouthe closes thisswitch. At 12 are shown a plurality of switches which are preferably ofa locking'type, that is, they remain closed'when operated until againoperated for opening. There is one of these switches provided for eachtransducer station on the ship. As each is closed a correspondingtransducer is placed in circuit so that the bridge ofllcer may controlfrom the least 'in part by the respective switches l2. Each of thesecontrol units consists .of two relays 62 and 60. The reference numeral[4 indicates generally the control. unit under the influence of theswitch II by means of which-the apparatus is controlled at the bridgefor operation either for talk-out over the microphone to or for talkbackover the transducers and the. loudspeaker 9. This control unit consistsof three relays 43, M and 45. It may be noted that the apparatus withinthe square I3 for the middle transducer is the same as that illustrateddirectly above and below it and hasmerely been shown in this form tofurther simplify the drawing.

One terminal of the microphone I0 is connected by wire l5 to thegrounded terminal of the volume control 20, which is illustrated ascomprising variable resistors and is also connected by wire l8 to theinput lead IQ of the amplifier 25. The other terminal of the microphonelil is connected by wire IE to the input terminal of the volume control20 as shown. The output terminal of this volume control is connected bywire 2| to the normally open fixed take other forms.

push-pull circuit.

contact of switch 22, which is a part of relay 44. The leads l5 and I6are bridged by a condenser li, for the purpose of properly equalizingthe talk-out frequency characteristic of this particular circuit. Themovable contact of switch 22 is connected by wire 23 to the other inputterminal of the amplifier 25, through variable impedance network 24.

The amplifier 25 is illustrated as an ordinary audio-frequency vacuumtube amplifier, which, as those skilled in the art will appreciate, mayReferring to the amplifier generally, it is'noted that the inputtransformer is indicated at IT. The lead from one terminal of the outputof this transformer to the control grid of the vacuum tube 26 includes acondenser 69, which will be referred to later. The circuits of the firstamplifier tube 26 are resistancecoupled to the circuits of the secondamplifier tube Zl', the output of which tube is coupled by v a.transformer to a pair of multi-electrode vacuum tubes 28 and 29connected in the well known The reference numeral F in all casesindicates the filament or heater leads of the vacuum tubes which wouldbe connected to the terminals F of the winding of the power packtransformer, all in accordance with well known practice. Theseconnections have not been completed for sake of simplicity. The outputof the amplifier is supplied to the output transformer OT, which isprovided with the leads 32 and 33. At.3l is indicated a full wave vacuumtube rectifier, which is used in conjunction with the usual power supplyfilter arrangements, etc., as is well known in the art.

' The leads 30 are to be connected to a suitable alternating currentsupply source which may be from an inverted rotary converter operatingfrom the power line of a ship. For emphasis, it is again noted that thisamplifier may take any one of a number of well known forms and is onlyto be difierentiated therefrom by the features which will be referred tolater.

The output of the amplifier appears between ground (wire 33) and wire 32which is connected to the normally open fixed contact of switch 36,which is a part of relay 43. This wire is also connected to the normallyclosed fixed contacts of switch 34 which is a part of relay 44 andswitches 31 and 38 which are parts of relay 45. The movable contact ofswitch 34 is connected by wire 35' to one terminal of the receiving orbridge loudspeaker 9. The other terminal. of this loudspeaker isgrounded by wire 33, which is also connected to both of the movablecontacts of switches 31 and 38 (relay 45) and to the normally open fixedcontact of switch 39 which is a part of relay 43. The movable contact ofswitch 39 (relay 43) is connected by a Y normally closed fixed contactof switch 36 (relay 43) is connected by wire 54 to the input terminal ofthe constant impedance volume control 53 shown as composed of variableresistors. The ground terminal of the volume control is connected towire l9, and the output terminal 0f the volume control is connectedthrough the condenser 52' by wire 52 to the normally closed fixedcontact of switch 22 which is a part of relay 44. The normally openfixed contact of switch 42 (relay 44) is connected by wire'5D to thenormally open fixed contacts of switch 40 which is a part of relay 43.The movable contact of switch 40 (relay 43) is connected to the lead Iof the direct current power source. The movable contact of switch 42(relay 44) is connected by wire 5| to one terminal of the winding of therelay 45, which includes switches 31 and 38. The other terminal of thiswinding is connected by wire 2 to the power source and wires 5| and 2are bridged by a suitable current reducing resistor, as indicated at 5|.The normally closed contact of switch 42 (relay 44) is connected by wire42' to each of the movable contacts of switches 65 which are apart ofrelays 60 in the control units l3. One of the fixed contacts of switch40 (relay 43) is connected by wire 48' to each of the normally openfixed contacts of the switches 65 (relays 60). The normally open fixedcontact of switch 4| (relay -44) is connected to the lead I of the powersource and the movable contact is connected by wire 48 to one terminalofthe winding of the relay 43, which includes switches 36, 39 and 40.The other terminal of this winding connects to the positive lead 2 ofthe current source.

Leads 48 and 2 are bridged by condensers 49 which act as time delaydevices to delay the release of the relay 43. One terminal of thewinding of the relay 44, which includes switches 22, 34, 4| and 42 isconnected to the positive terminal 2 of the current source, and theother terminal is connected by wire 41 to one terminal of push buttonswitch The other terminal of switch H is connected to wire I of thedirect current power source. One contact of each of the switches I2 isconnected by wire 55 to wire' I of the direct current power source. Theother contact of each of these switches is connected by wires 66respectively to the movable contact of switch 63 which is a part ofrelay 62 and to the normally closed fixed contact of switch". Thenormally open fixed contact of switch 58 is connected to the directcurrent power source wire I, which is also connected to the movablecontact of switch 64, which is a part of relay 62 and to the normallyclosed fixed contacts of switch 68 which is a part of relay 60. Thepower, lead 2 is connected to the commonterminal of two opposingwindings of the relay 62 and by wire 6| to one terminal of the windingof the relay 60. The other terminal of the winding of the relay 60 isconnected by wire 59 to the movable contact of switch 58 which is a partof relay 62. The normally open fixed contact of switch 63 (relay 62) isconnected through a resistor as shown to the remaining terminal of theright hand winding of relay 62 while the remaining terminal of the lefthand winding of this relay is connected through a resistor to thenormally open fixed contact of switch 64 (relay 62) and to the'normallyclosed fixed contact of'switch 66 which is a part of relay 60. Themovable contact of switch 66 (relay 66) is connected by wire 66' to oneterminal of the transducer 12'. The movable contact of switch 68 (relay60) is connected by wire 68 to the other terminal of the transducer I2through a condenser |2b which acts as a blocking condenser for directcurrent; A push-button switch |2a is provided for connecting the leads66' and 68' of the transducers together for turning the station on fromthe outside. It may be' noted .that the connections in each of thecontrol units l3 are the same as those just described.

The network 24 connected in the lead 23 consisting of a condenseradjustable to three values of capacitance, shunted with a resistor, andthe condenser 69, Figure 2, are provided so that the over-all responseof the network and amplifier in combination with the transducers aspreviously described will be non-linear but having a rising trend withincreasing frequency and rapid atten Q uation below about 200 cycles persecond and an operating range up to about 3,000 cycles per second. It isto be noted that the volume controls 20 and 53 are used alternately, thevolume control 20 being used for talk-out and the volume control 53being used for talk-back. The circuit connections as' will be describedin connection with the operation are such that the change over from onevolume control to the other is accomplished automatically to provide theproper volume level or to accomplish good speech intelligibility whenthetransducers are used as talk-back units. The

talk-back volume control is adjustable to control the talk-back signallevel in order to secure the proper signal strength for maximum earsensitivity under the operating conditions likely to be encountered. Thecondenser 52' in series with the talk-back volume control 53 reducesthe-low frequencies further for talk-back.

In describing the operation of this system, it is first to be noted thatall switches and connections are shown in their normal positions, thatis when the system is not in use, although the switch 3 is closed sothat energy is available to put the system in use. If it be assumed thata bridge ofiicer wishes to talk-out through one or more of thetransducer stations,.he first closes switch l2 corresponding to thosestations he desires to operate. He then closes switch I, with the resultthat current flows from wire I through switch If wire 45, relay u, backto wire 2. The result is that relay 44 is operated and themovableconswitches I2 have been closed.

as is clear from Figure 1.

, t 2,255,617 tacts of switches 22, 34, 4I and42 are moved over lead I9is already connected to the other ter-.

minal of this microphone. The closing of switch 4I supplies currentthrough wire I, switch 4|, wire 48, to relay 43, and from there throughwire 2 back to the'direct current power source. Since there is verylittle resistance in series with the parallel circuit of relay coil 43and condensers 43, the relay operates immediately and switches 40, 38and 35 are operated. The operation of switch 34 disconnects wire 35 fromwire 32 so that the bridge loudspeaker is out of circuit.

As soon as switch 43 is operated current flows.

from wire I, through the normally open contact of switch 40 (why 43),wire 50, normally open contact of switch 42 (relay 44), the movablecontact wire 5|, relay 45, and back to direct current power sourcethrough the lead 2. The energiration of the winding of relay 45 opensswitches 31 and 38, which are simply in parallel. The opening of thesecircuits removes a short-circuit across the output terminals 32 and 33of the amplifier. The operation of switch 36 which is a part of relay 43connects the output lead 32 of the amplifier to wire 35' and switch 39also a part 'of relay 43 connects the other output lead '33 to wire 39'.Wires-36' and 39' run respectively to the normally open contacts of theswitches 66 and 88 of each of the relays 60.

It will be recalled that one or more of the Assuming that the uppermostswitch I2 has been closed, current flows from lead I, through lead 55,upper switch I2, wire 56, switch 58 (relay 53) the winding of relay 60,and back to the lead 2 through wire 6|. This operates relay 60, closingswitch 65 and operating switches 65 and 68. The operation of switches 86and 68 connects the corresponding transducer I2 to the leads 36 and 39',The same operation occurs in each control unit I3 for which thecorresponding switch I2 has been closed.

The system is now set up for operation for talk-out, that is, the bridgeofiicer speaking over the microphone I supplies speech energy to theamplifier, the output of which is supplied to the transducers I2 whoseswitches I2 are closed. During all the time the bridge oificr talks heholds switch II closed. In order to be talked to, that is, in order fortalk-back operation to occur, as soon as he finishes. speaking orissuing a command, he releases switch II. The opening of switch I Ibreaks the, circuit to relay 44 with the result that all the switchescontrolled by it move to the position shown in Figure 1. Thus the bridgeloud-speaker 9 is connected throughswitch 34 which is a part of relay 44to output lead 32 and, since it is always connected to output lead 33,it is ready to be operated, except that the release of relay 44 alsocaused relay 45 to release and short-circuit the output of the amplifiertemporarily.

Input lead I9 is connected through switch 39 (relay 43) to lead 39 andinput lead 23 is connected through switches 22 and 36 to lead 36'. Atthis time the network 53 is in circuit and netn at switch 4:. Thusswitches 31 and as of relay 45. close and momentarily short-circuit theloudspeaker 3, preventing the emission from the loudspeaker of thevarious noises incident to the change-over, which has been foundimportant in that the elimination of these noises is desirable. It hasbeen found that if these noises are permitted to reach the loudspeaker 9the person thereat is initially distracted as the speech comes over theloudspeaker. Practice indicates that the elimination of this distractionserves for more accurate communication. The across the speaker onlyexists momentarily, as will be explained later.

The release of relay 43 sufliciently to .permit the switches controlledthereby to move back from operated position to the position shown inFigure 1 is momentarily delayed by reason of the time required vfor thecondensers 49 to discharge through the relay winding and it is duringthis period that the short-circuit on the bridge loudspeaker at switches3'I and 38 of relay 45 occurs.

However, as soon as switch 40 moves back to the position shown, currentflows from wire I, through wire 40', through closed switch 65, wire42',,the normally closed contacts of switch 42, wire 5|, relay 45, andback to the direct current This causes switches, 31 and 38 of relay 45to open and break the short-circuit. Relay 63 remains operated, holdingswitch 55 closed and the transducer connected to the leads 35 and 39'.

In addition to connecting the loudspeaker 9 to the output leads of theamplifier, the release of the relay 44 causes the transducers to beconnected to the input of the amplifier at switches 36 and 39 of relay43. The system is now ready for talk-back in a manner which it isbelieved will be apparent. This change in connections substitutes thetalk-back volume control 53 for the talk-out volume control 20 so thatthe gain of the amplifier system ischangedfor talkback in accordancewith the description previously provided. The condensers I2b in additionto acting as blocking condensers assist in providing the proper amountof equalization for the transducers for maximum speech intelligibilityboth for talk-out and talk-back." These con-' densers, if desired, maybe variable as may be the other modifying elements of the circuits toadapt them for operation under varying conditions. I r

The operation of the system will now be described for a case wheretalk-back is desired to the bridge, the conversation to initiate at thetalk-back end. Assuming switchesI2 are open,

work 20 is out of circuit, switch 22 being in the pos tion shown.

The release of relay 44 also'breaks the circuit to relay 45 at switch 42and the circuit to relay as they would normally be, the person desiring"to talk-back to the bridge, at one of the trans- The closing oi switchI2a, say for the upper transducer station in Figure 1, causes current toflow from wire I, through the normally closed contacts of switch 63which is a part of relay 50, switch I2a, back through the normallyclosed contacts of switch 56, also 'of relay 60, to and through the lefthand winding of relay 62 and back to lead 2. The energization of theleft short-circuit hand winding of relay 62 is sufllcient to causeoperation of the three switches 68, 63 and 64 controlled thereby.Current fiows through lead I, the normally open contacts of switch 58,wire 59, relay 60, and wire 51. back to wire 2. The closing of switch 64completes a holding circuit for this winding from wire I, through switch64 to the left hand winding of magnet 62 and back to the direct currentpower lead 2 so that even though switch I2a is immediately opened theleft hand winding of relay 62 remains energized.

The operation relay 60 connects the transducer with the wires 36' and39' as before'through the normally open contacts of switches 66 and 68.The closing of switch 65 causes current to flow from lead I through thenormally closed contact of switch 40, which is a part of relay 43, wire40, switch 65 (relay 60), wire'42', normally closed contacts of switch42, wire relay 45, and back to the lead 2. Thus the short-circuit istaken oiT the bridge loudspeaker at'switches 31 and 38 of relay 45.

The system is now ready for talk-back. The bridge oflicer, in order totalk-out, closes the corresponding station switch I2 and as he talksholds switch II closed, which of course operates the control unit I4, aspreviously described. The closing of switch II also supplies currentfrom the lead I, wire 55, switch I2, wire 55, closed switch 63 of relay62 and the right hand winding of relay 62, back to the power lead 2.Thus the right hand winding of the relay 62 is energized in the oppositedirection to the other winding so that the relay releases and switches58, 63 and 64 assume their normal positions. Relay 60 remains operatedas long as switch I2 is closed however as previously described. It istherefore obvious to those skilled in the art that when the system hasbeen put in operation by pressing the outside push button I2a, it may beshut ofi by the bridge oflicer by operating and then releasing hisswitch I2.

Figure 3 showsdiagrammatically a modified circuit which differs in themain from that of the circuit of Figures 1 and 2 in that part of theequalization or control means by which good intelligibility of speech issecured and the volume thereof controlled is shown inserted in thereceiving speaker circuit in place of the input circuit of theamplifier, as in the former circuit.

In Figure 3 some of the reference numerals used in the previous figuresare employed to designate the same or substantially similar parts forthe purpose of aiding in interpreting the disclosure. ln this case thedirect current power supply circuit is represented by the negative leadI. and the positive lead 2 and controlled by the main switch 3. Thenegative side of the circuit is connected by wire through a choke coil1| to one side of the push button switch H. The other side is connectedby wire 12 to the winding of relay 13, the other terminal of which isconnected by wire 14, back through the choke coil 14' to the other sideof the-main switch 3. Thus when switch II is closed, relay 13 isoperated and the switches 6, 80, 84 and 88 move from their positionsshown to their other positions.

At this point it may be noted that the heaters or filaments FI to F-6,inclusive, for the rectifier tube 11 and amplifier tubes I22 and I26 arein series across the line through the tapped resistor I I9 when switch 3is closed. Thus current flows from wire I0 through wire 125, the wholeof resistor H9, filaments FI to F6, and wire 14 to the other side of theline. Thus when the system is in operation the heaters of all the tubesare at operating temperatures and floating across the line. Here it maybe noted that the rectifier 11 is a half wave vacuum tube rectifier witha resistance-capacitance filter and vacuum tubes I22 and I26 are theusual amplifying tubes with their associated circuits coupled to form acascade amplifier with which the tubes I22 are associated in a push-pullcircuit. The choke coils '1I and 14' are simply for filtering purposesto keep the variations in screen and plate potential of the output tubesfrom reacting on the input. The filaments F--I to F6 are all shownassociatedtogether to simplify the circuit but they will, of course,occupy their respective positions in the vacuum tubes as represented bythe same symbols and reference numerals with respect to each tube. Thereference numeral I00 represents generally the audio-frequency amplifieremployed in this system, which includes the input transformer IT and theoutput transformer OT.

The high impedanc primary terminal of the input transformer is connectedby wire 18 to the normally open fixed contact of switch 60 which is apart of relay 13. The movable contact of this switch is connected bywire 19 to the movable contact of switch 84, also on relay 13. Thenormally open fixed contact of switch 84 (relay 13) is connected by wire81 to one terminal of the bridge microphone IO. The other terminal ofthis microphone is connected by wire 82 to ground, as shown at 83, as isone terminal oi the bridgeloudspeaker 9. The other terminal of thisloudspeaker is connected through an adjustable impedance network showncomprising inductance, resistance and capacity, and a volume controlII'I comprising variable resistance with wires 83 and H6. Wire 6 isconnected to the normally closed fixed contact of switch 6 which, is apart of relay 13. The movable contact of this switch is connected bywire 85 to one of the terminals of the output transformer OT of theamplifier. The other terminal of this transformer is grounded as is theother terminal of the input transformer as shown. The normally closedterminal of switch 80 (relay 13) is connected by wire I M to the lowimpedance taps on the input transformer IT. The normally open fixedcontact of switch 6 (relay 13) is connected, by wire 81 to the normallyopen fixed contact of switch 88 which is also a part of relay 13. Thenormally closed fixed contacts of switches 88 and 84 on relay 13 areconnected together. The movable contact of switch 89 (relay 13) isconnected by wires 94 and 69 to the normally open fixed contacts ofswitches 95 and 93 on relays I05 and I06. The movable contacts ofswitches 93 and 95 of the relays I06 and I05 are connected respectivelyby the wires 90 to one terminal of each of the transducers I2. The otherterminal of each of the transducers is con nected back through thenormally closed switches 12a by wires M to the movable contacts ofswitches 96 and 91 of the same relays.

The normally open fixed contacts of switches 96 and 91 on relays I05 andI06 are connected by wires 82 and 92 to ground 83. The normally closedfixed contacts of switches 93 and 95 on relays I06 and I05 and one endof the tapped resistors I09 and H0 are all connected by the wire II4 tothe negative side of. the power supply through main switch 3. Themovable contacts of switches I04 and98 on relay I06 and switches 69 andIN of relay I05 are connected by wire I24 to one end of the tappedresistor H9. The

total current. flows through this circuit. so that normally open fixedcontacts'of each of the switches I04 and 08 on relay I08 are connectedby wire I03 to the normally open fixed contacts of each of the switchesMI and I02 on relay I05, and by wires I23 and I23 to the tap on resistorH9. One terminal of the wlndingof relay I06 is the relay I06 is-normallynot operated. However,

connected by wire III to the movable contact of the lower station switchI2. The normally engaged contact of this switch is connected to wireIll. The normally disengaged contact of this switch and the other switchI2 are connected to gether, and tothe positive terminal of the powersupply through wires I4, choke I4, and main switch 3. upper switch I2 isalso connected to wire I0. The other end of resistance I09 is connectedby wire III to the normally disengaged contacts'of the switches I2. WireIII is connected as shown to wire I4. A tap connection from resistanceI09 is'connected by wire I08 to the other terminal of the magnet I06.Wire III is also connected to a resistance II 0,.the other terminal ofwhich is connected by wire H3 to wire'II4 which is connected to one ofthe fixed contacts of switch 95. The other fixed contact of this switchis connected by wire 94 with the movable contact of switch 88. Themovable. contact of the upper switch I2 is connected bywire II to oneterminal of the magnet I05. The other terminal of this magnet isconnected to wire II2 which comprises a tap on resistance H0 and to oneof the fixed contacts of switch 96.

if a person at the transducer station pushes the button I2'amomentarily, this will force the current to flow through the relay coilI06 causing the operation of the relay.

This relay is also operated when the lower station switch I2 is turnedon. The circuit in this case is as before through the choke coil I4,wire III, the normalfy open contact of switch I2, the movable contact ofthe same switch, wire I01,

. the winding of relay I06, wire I08, back to the The normally engagedcontact of the In describing the operation of this circuit it will beassumed that all the switches are in their ofi" position as shown,except switch 3 which must necessarily be on. Current flows through lead2 from the power source through switch 3,wire 14, the heaters F6, F5,F4, F3, F2, PI of the amplifier and rectifier .tubes, one section ofresistor N9, the other two sections of resistor H9 in parallel, wireI25, and back through switch 3 and wire I to the negative terminal ofthe power source. This heats the filaments or heaters of the vacuumtubes sufiiciently so that they are ready for operation. It will benoted that the voltage drop across one section of resistor H9 isconnected in series with the grid bias for the output tubes I22. Thevalue of this voltage drop is sufiicient to bias these tubes to cut-ofiso that no plate current flows; this is the normal or standby conditionfor this'amplifier. Current also fiows from the positive terminal of thepower source through wire 2, switch 3, wire I4, choke coil I4, rectifiertube 'I'I, to the first two stages of the amplifier comprised in thedouble tube I26; Current also flows as described above to and throughthe choke coil I4, then through wire II I to one end of eachof thetapped resistors I08 and H0. The current passing through resistor I09flows through the normally closed contact of switch 91, which is a partof relay I06, to the movable contact of the same switch, through wire 9I, normally closed push button switch I2a, transducer I2, wire 90, themovable contact of switch 1 93 on the same relay, the normally closedfixed contact of the same switch, and back through wire H4 and switch 3to the negative terminal of the power supply I. the transducer circuitthrough wire I08, the winding of relay I06, the movable arm of switchI2, the normally closed contact of the same switch, choke coil 1I, wireI0, and back through switch 3, to the negative terminal of the powersupply I. The resistance of the winding of the relay is so much higherthan the resistance of the transducer circuit that only a negligibleportion of the There is a parallel path around" tap on resistor I09. Itwill, of course, be apparent to those skilled in the art that relay I05may be operated in the same way'as relay I06. The

operation of either or both of these relays short circuits one part ofresistor II9 through wires I23, I23, switches MI and I02 in parallel orI04 and 98 in parallel, wires 89 and I24. The short circuiting of thispart of resistor II9 removes the cut-off bias from the grid circuitofthe power amplifier tubes I22 and simultaneously increases the filamentcurrent to the normal value, this,

by wire I2I, the normally closed fixed contact of q witch 80, themovable contact of the same switch. wire IS, the movable contact ofswitch 84, the normally, closed fixed contact of the same switch, thenormally closed fixed contact of switch 88,

the movable contact of switch 88, and wire 89,

to whichever of the transducers I2 is connected as described above. Theoutput transformer of the amplifier I00 is connected through the movablecontact of the switch 6, the normally closed fix'ed contact of the sameswitch, wire I I6, volume control and adjustable impedance network II!to the bridge loudspeaker 9.

As will be seen from the diagram, one end each of the input transformerIT, the output transformer OT, the transducer I2, and the bridge speaker9 is grounded. Thus the necessary circuits for talk-back operation areset up. I

When th push button II associated with the microphone I0 is operated,current flows as above described, to and through the choke coil I4, wireIII, the winding of relay I3, wire I2, the contacts of switch II, chokecoil II, wire I0, and back to the negative side of the power supplythrough switch 3 and wire I. This causes relay I3 to operate and theswitches 6, 80, 84, and 68 to move to the positions opposite frorn'thoseshown in the diagram. The circuits for talking out are now set up asfollows: The microphone I0 is connected by wire 8|, the normally openfixed contact of switch 84, the movable contact of the same switch, wireI9, the movable contact of switch 80, the normally open fixed contact ofThe condenser it has its positive side con nected through rectifier Hand wire Hi to one side of the relay coil 33, and its negative sideconnected directly to the other side of said relay coil. This condenserhas a value of capacity sufficiently large to delay the release of therelay 13 when the push button ii is released. This slowing up of therelease of relay l3 minimizes undesired clicks or thumps in the bridgeloudspeaker which would otherwise have a distract ing effect.

The. condenserv 520 in combination with the volume control shownconnected to the tube. I26, attenuates the undesired low frequencycomponents of speech and extraneous noises in ord'er. to increase theemphasis in the higher vowel, voiced consonants and sibilant regions. Inthe talkback circuit, additional control over the irequency response ofthe amplifier, as well as its gain, is provided by the volume controland adjustable impedance network iii, the controls of which are mountedon the bridge so that the bridge officer may adjust the talk-back gainand frequency response to suit operating conditions.

What We seek to secure by United States Letters Patent is:

1. An audio-frequency signaling system including a microphone, aloudspeaker, a transducer, a multi-tube vacuum tube amplifier, means foralternately connecting the microphone to the input of the amplifier andthe transducer to the output thereof, or the transducer to the input ofthe amplifier and the loudspeaker to the output of the amplifier, andcontrol means adjacent the microphone and the transducer whereby twowayaudio-frequency communication mat be initiated from either point.

2. In an audio-frequency communication system the combination includinga microphone and a loudspeaker at a central station, a transducer at areceiving station, an amplifier, and means controlled from either thecentral station or the receiving station for connecting the microphoneto the input of the amplifier and the transducerto the output of theamplifier, or the transducer to the input of the amplifier and theloudspeaker to the output of the amplifier.

3. In an audio-frequency communication system, the combination includinga microphone and a loudspeaker at a central station, a transducer at areceiving station, an amplifier, means controlledfrom either the centralstation or the receiving station for connecting the microphone to theinput of the amplifier and the transducer to the output of theamplifier, or the transducer to the input of the amplifier and theloudspeaker to the output of the amplifier, and an impedance networkconnected in the input of the amplifier whereby the over-all response ofthe system is non-linear and has a rising frequency response withincreasing frequency.

4. In a two-way audio-frequency communication system, the combinationincluding a microphone and loudspeaker situated at a central station, atransducer situated at a receiving station, an amplifier, an impedancenetwork connected to the input of the amplifier, and means at thecentral station for connecting the microphone to said network and thetransducer to the output of the amplifienand means at the receivingstation for connecting the transducer to the net work and th loudspeakerto the output of the amplifier, said network and amplifier incombination having a non-linear response and the respouse having arising trend with increasing frequency.

5. In a two-way audio-frequency communication system the combinationincluding a microphone and loud-speaker situated at a central station, atransducer situated at a receiving station, an amplifier, animpedance'rietwork connected to the input of the amplifier, means at thecentral station for connecting the microphone to said network andtransducer to the input of the amplifier, means 'at the receivingstation for connecting the transducer to the network and the loudspeakerto the output of the amplifier, said network and amplifier incombination having a non-linear response and the response having arising trend with increasing frequency, and means for short-circuitingthe transducers dur ing change-over.

6. In a two-way audio-frequency transmission system, the combinationincluding a microphone and a loudspeaker, a transducer, an amplifier, apair of volume controls, means for connecting the microphone to theinput of the amplifier, the transducer to the output of the amplifier,and one of said volume controls in circuit for speech transmission inone direction, and means for connecting the transducer tothe input ofthe amplifier, the loudspeaker to the output of the amplifier, and theother volume control in circuit for speech transmission in the oppositedirection.

7. In a two-way audio-frequency transmission system, the combinationincluding a microphone and a loudspeaker, a transducer, an amplifier,

a pair of volume controls, means for connecting the microphone to theinput of the amplifier, the transducer to the output of the amplifier,and one'of said volume controls in circuit for speech transmission inone direction, means for connecting the transducer to the input of theamplifier, the loudspeaker to the output of the amplifier, and the othervolume control in circuit for speech transmission in the oppositedirection, and means for short-circuiting the transducer duringchange-over.

8. A two-Way audio-frequency communication system including a centralstation having a microphone and a loudspeaker, a remote station having atransducer, an amplifier, input and output circuits for the amplifier, avolume control in the output of the amplifier, and means at the centraland remote stations for connecting at will either the microphone or thetransducer to the input of the amplifier and the transducer andloudspeaker to the output of the amplifier respectively.

9. A two-way audio-frequency communication system including a microphoneand a loudspeaker, a transducer, an amplifier, input and output circuitsfor the amplifier, a volume control in .the output of the amplifier,means for connecting at will either the microphone or the transducer tothe input of the amplifier and the transducer and loudspeaker to theoutput of the amplifier respectively, and impedance means in theamplifier circuit for imparting thereto a limited frequency range ofnon-linear characteristics. I

10. A two-way audio-frequency communication system including amicrophone and a loudspeaker, a transducer, an amplifier, input andoutput circuits for said amplifier, means at one point for connectingeither the microphone or the transducer to the input circuit of theamplifier and the loudspeaker or the transducer to the output of theamplifier, whereby two-way communication may be accomplished andcontrolled from one point, and means at the transducer station forinitiating two-way conversation.

11. A two-way audio-frequency communication system including amicrophone and a loudspeaker, a transducer, an amplifier, input andoutput circuits for said amplifier, and means at one point forconnecting either the microphone or the transducer to the input circuitof the amplifier and the loudspeaker or the transducer to the output ofthe amplifier, whereby two-way communication may be accomplished andcontrolled from one point, and a pair of volume controls alternativelyand automatically connected to the input circuit of the amplifier foreither direction of communication.

12. A two-station long distance two-way audioirequency communicationsystem including a microphone and an-amplifier, a transducer havingdirectional characteristics in a dimensional sense, with an increasingresponse with increasing frequency, a non-linear amplifier and input andoutput circuits for the amplifier, and control means at each station forconnecting either the microphone or the transducer to the input circuitof the amplifier and the transducer or loudspeaker respectively to theoutput circuit of the amplifier.

13. A long distance two-way audio-frequency communication systemincluding a microphone and an amplifier, a transducer having directionalcharacteristics in a dimensional sense with an increasing response withincreasing frequency, an amplifier and input and output circuits for theamplifier and control means for connecting either the microphone or thetransducer to the input circuit of the amplifier and the transducer orloudspeaker respectively to the output circuit of the amplifier, saidamplifier and input circuit having a non-linear response and a limitedfrequency range.

14. A long distance two-way audio-frequency communication system havingtalk-out and talkback stations, a microphone and a loudspeaker at thetalk-out station, a transducer at the talkback station, an amplifier,talk-out and talkback volume control networks, and means at speaker atthe talk-out station, a transducer at the talk-back station, anamplifier, talk-out and talk-back volume control networks, and means atthe talk-out station for connecting the microphone to the input of theamplifier through the talk-out network and transducer tothe output ofthe amplifier or the transducer to the input of the amplifier throughthe talk-back network and the loudspeaker to the output of theamplifier, and means for short-circuiting the transducer duringchangeever from talk-out to talkback or talk-back totalk-out.

16. A long distance two-way audio-frequency communication systemincluding a talk-out station having a microphone and loudspeakerthereat, a plurality of talk-back stations each having a transducerthereat, an amplifier, means at the talk-out station for connecting themicrophone to the input of the amplifier and one or more ofthe-transducers to the output of the amplifier, and means at eachtransducer station for connecting the transducer thereat to the input ofthe amplifier and the loudspeaker to the output thereof...

FRED W. NICKERSON. WILLIAM C. MOORE. HERBERT BARNEE,

